Modern passenger or cargo ships have in general a variety of openings lying under water. Such openings serve, for example, for the suction of cooling water from the fairway water and for the reinjection of the heated cooling water into the fairway water of the ship. Furthermore, to improve the maneuverability of a ship, for example, a thruster can be provided in the bow- and/or stern-region. With favorable flow- and weather-conditions thrusters can make unnecessary a cost-intensive use of tugboats in docking maneuvers, in particular for the transverse movement of the ship. However, such thrusters require a transverse channel completely penetrating the ship hull below the waterline in the bow- or stern-region, whereby two extensive opposing openings arise.
Due to the end-side openings of the transverse channel, turbulences arise in the water that lead to an increase of the flow resistance of the hull in normal driving operation of a ship. This in turn results in a now unacceptable increase in fuel consumption. A high significance is associated with this fact due to the often high travel speeds of cargo and passenger ships, which can fall in a range of 20 knots.
In order to reduce the flow resistance of a ship hull in normal driving operation, circular butterfly valves are known, for example, for the closing of the openings of a transverse channel of a thruster. Due to the rotating of the butterfly valves respectively disposed in the region of one of the two opposing openings of the transverse channel, in normal driving operation of the ship the openings can close nearly flush with the outer hull skin. In the maneuvering operation of the ship the butterfly valves are opened by rotating about their longitudinal center axis by 90°. In the fully opened state the butterfly valves are oriented parallel to the longitudinal direction of the transverse channel, so that a water flow generated by a drive propeller of the thruster can pass the butterfly valves largely unhindered due to their low material thickness in relation to the diameter of the transverse channel.
However, many years of experience with such butterfly valves have shown that in particular their bearing points generally disposed diametrically in the opening are subject to a high mechanical load due to the propeller slipstream of the thruster, external flow forces, and the impact of waves. Furthermore, the bearing points of the butterfly valves are permanently exposed to the corrosive fairway water. In themselves or in combination with one another, in the extreme case all influencing factors can lead to the complete failure of such a butterfly valve.